The recovery of low boiling gases such as hydrogen, helium and neon from gas mixtures containing both low boiling and higher boiling gases such as nitrogen, oxygen, argon, C.sub.1-2 hydrocarbons, carbon monoxide, krypton, and xenon are well known and have been widely practiced in the art. Examples of feed streams containing hydrogen, helium or neon for recovery from higher boiling gas include gases from nitrogen rejection units used to recover to C.sub.1-2 hydrocarbons from natural gas streams, nitrogen from air and carbon monoxide production from synthesis gas. Representative patents which describe various processes for the recovery of the low boiling light gases from higher boiling gases include the following:
U.S. Pat No. 3,269,130 discloses a process for the separation of gaseous mixtures for generating hydrogen and nitrogen streams which then may be used for the synthesis of ammonia. The high boiling components are separated from the low boiling nitrogen and hydrogen components by charging a cooled liquid to a conventional scrubbing column equipped with vertically disclosed plates. A scrubbing liquid which comprises deeply subcooled nitrogen is introduced to the top plate of the scrubbing column wherein a gas mixture rich in the low boiling hydrogen-nitrogen components is generated. Liquid is withdrawn from the bottom of the column and contains the high boiling component and scrubbing liquid.
U.S. Pat No. 4,675,030 describes a variation to the cryogen processes for recovering helium and nitrogen. A gas mixture containing helium and higher boiling components such as carbon dioxide is cooled to remove water therefrom and then charged at superatmospheric pressure to a helium-permeable, oxygen/nitrogen-impermeable membrane. The permeate contains substantially pure helium. The impermeate, because it still contains some helium, is fed to a second membrane system. The permeate from the second membrane which contains some helium is recycled as feed to the first substantially helium-permeable, oxygen/nitrogen-impermeable membrane. The impermeate then is rejected from the system.
U.S. Pat No. 3,260,058 discloses a cryogenic process for the recovery of helium from gases. The process is representative of some of the early processes for recovering a low boiling gas such as helium from a nitrogen containing stream. In that process, fuel gas or residue gas containing helium is cooled and then passed to a series of flash columns stacked one on top of each other. A cooled feed is charged to a first flash column wherein liquid and vapor are generated. Liquid is taken from the bottom of the flash column, expanded and charged to an intermediate portion of a second adjacent flash column. The vapor fraction from the first flash column is removed and let down in pressure. The liquid from the bottom of the second flash column is expanded and charged to the third flash column which sits atop of the second column. The vapor fraction from the second flash column is removed and expanded and combined with the vapor fraction from the first column. The process is repeated for flash columns 3, 4, 5, and so on wherein the liquid from the bottom of the next adjacent column is expanded and charged as feed to the next adjacent column. Although the process is effective for generating and recovering low boiling gases such as helium or hydrogen with high recovery, it suffers a power penalty because of the repeated expansions and liquefaction steps required to effect separation and concentration of the light components.
French Patent 1,360,323 discloses a process for recovering helium from gas mixtures comprised principally of nitrogen and methane. The separation is accomplished by generating liquid fractions which are then separated from the residual gas fractions enriched in helium. The residual gas fractions are reliquified to eliminate residual nitrogen. Liquefaction of the residual gas enriched in helium is effected by condensation with liquid nitrogen under low pressure. Final purification of the helium is obtained by passage and contact with an adsorbent mass at low temperature.
U.S. Pat No. 4,701,200 discloses a process for recovering helium gas from a nitrogen rejection unit. In the process a gas mixture containing nitrogen, methane and helium is cooled, expanded and charged to a high pressure column wherein a liquid and/or vapor fraction are formed. The vapor fraction is partially condensed and the resulting liquid fraction is separated in a phase separator. The vapor fraction obtained from the separator is again partially condensed and the vapor fraction separated from the liquid fraction in a second separator. That vapor fraction is then warmed against process streams and charged to a pressure swing adsorption unit for further purification. The liquid from the high pressure column is expanded and charged as feed to a low pressure column. Crude liquid from the low pressure column is warmed against the vapor stream obtained from the high pressure column in order to effect partial condensation. The phases are separated. The liquid is pumped to a higher pressure and the vapor then is charged to the bottom of the low pressure column as feed. An overhead vapor stream is taken from the low pressure column and it comprises essentially nitrogen.
U.S. Pat. No. 4,758,258 discloses a cryogenic process for separating helium from helium-bearing natural gases. To accomplish this separation, a series of steps effecting removal of boiling components is performed. A partially condensed natural gas feed is introduced to a first fractionation zone wherein a vapor phase comprised predominantly of helium and nitrogen and a liquid phase containing higher boiling hydrocarbons are obtained. The vapor phase from this column is partially condensed and then introduced into a second fractionation zone wherein a second vapor phase comprised essentially of helium and nitrogen is generated. At this stage essentially all of the hydrocarbons have been removed from the second vapor stream. This process is repeated several times removing any residual amounts of methane and thereby concentrating the helium in the vapor phase.